The invention relates generally to vehicle drive systems, and more specifically to battery powered drive systems such as those used in battery powered electric vehicles, hybrid electric vehicles, or plug-in hybrid electric vehicles.
Electric vehicles and hybrid electric vehicles are typically powered by one or more energy storage devices, either alone or in combination with an engine (e.g., internal combustion engine, turbine engine, etc.), with such energy storage devices including one or more of batteries, ultracapacitors, flywheels, or a combination of these elements in order to provide sufficient energy to power an electric motor. In pure electric vehicles, the one or more energy storage devices power the entire drive system, thereby eliminating the need for an engine. Hybrid electric vehicles, on the other hand, include energy storage device power to supplement power supplied by an engine, which greatly increases the fuel efficiency of the engine and of the vehicle.
A DC/DC converter is widely used in hybrid and electric vehicle systems as an interface between a battery and the motor drive DC link. It has been proposed previously to use more power dense energy storage like an ultracapacitor in combination with the battery to improve the transient power capability of the drive system, which enables reducing the overall system cost by reducing the power requirement of the battery and the DC/DC converter. However, one drawback of existing arrangements of a battery and ultracapacitor in a vehicle drive system is that the ultracapacitor is always connected to the DC link and the battery. Due to the large capacitance of the ultracapacitor, the voltage of the DC link capacitor cannot be changed in a short time period. Therefore, in comparison to a more common system that does not make use of an ultracapacitor, it is difficult to optimize the system efficiency by reducing the DC link voltage when the load power demand is low.
Therefore, it is desirable to provide an electric and/or hybrid electric propulsion system in which a fast variation of the DC link voltage may be achieved so as to improve system efficiency. It would be further desirable for such DC link voltage to be controllable without affecting the power capability of the system and for the ultra capacitor to provide transient power (both directions), so that the size of the battery and DC/DC converter can be minimized.